This invention relates to improved power operated actuators for moving a valve or other controlled unit in correspondence with variations in an input signal delivered to the mechanism.
An actuator embodying the invention is of a known general type including a piston and cylinder mechanism or equivalent structure utilized in conjunction with a pump or other source of pressurized fluid delivering such fluid under pressure to one side of the piston for power actuating the piston to a desired setting. In prior arrangements of this type it has been proposed that the pressure delivered to the cylinder chamber be varied by discharging the pressure fluid through a nozzle and against a flapper valve element positioned opposite the nozzle and movable relative thereto to controllably vary the rate of fluid flow from the nozzle and thus the pressure applied to the cylinder chamber. A force motor operated by an electrical input signal may actuate this flapper valve, with a feedback spring or other feedback means being connected between the piston and the flapper valve to counterbalance the force exerted by the force motor when the piston has reached a predetermined desired setting. The fluid pressure induced movement of the piston has in prior arrangements been resisted by a spring in a manner resulting in displacement of the piston from a predetermined normal position through a distance dependent upon the pressure applied to the cylinder chamber.